Since long-haul transmission in optical communication suffers reduction of signal light power due to a loss during propagation through a transmission line, the optical signal needs to be amplified by a repeater, so as to compensate for the loss. Optical amplifiers are known as devices to be used for this repeater.
Examples of the optical amplifiers include optical fiber amplifiers which use a rare-earth doped optical fiber as an amplifying medium, semiconductor optical amplifiers which use a semiconductor as an amplifying medium, Raman amplifiers which make use of stimulated Raman scattering of optical fiber, and so on.
In general, for controlling the optical output power of the optical amplifier or the gain in the optical amplifier, it is necessary to monitor each of output-signal-light power (signal light power after amplification) and input-signal-light power (signal light power before amplification). Various light power detection circuits to monitor the signal light power allocated in the optical amplifier are also used for detecting reduction of signal light power and increase of the reflected signal light power due to disconnection of optical fiber cord, and other abnormal conditions.
For example, Shouichi SUDO, “Erbium-Doped Fiber Amplifiers,” (2nd impression of 1st edition), The Optronics Co., Ltd., pp. 96-98, Apr. 20, 2001 (Non Patent Literature 1) describes the fundamental configuration of an Erbium-doped (Er-Doped) fiber amplifier (EDFA) which is a type of optical amplifier. Specifically, the EDFA is composed of an Er-doped optical fiber (EDF) as a gain medium for the amplification, a pumping light source which supplies pumping light for pumping of the EDF, a WDM coupler which multiplexes the pumping light and signal light, and an optical isolator which prevents the EDF from oscillating due to the counter propagated lights to the EDF.
For performing a control to keep the gain (constant gain keeping control) in the EDFA as described above, partial lights of the input signal light and output signal light branched by tap couplers are converted into electric signals by respective photodiodes (each of which will be represented hereinafter by PD) and the result of a comparison between the obtained electric signals is fed back to a control of the drive current of the pumping light source. For performing a control to keep the output signal power (constant output signal power keeping control) in the EDFA as described above, partial light of the output signal light branched by a tap coupler is successively converted into electric signals by a PD, and the result of a comparison between successively-obtained electric signals is fed back to the control of the drive current to the pumping light source. PD is a photoelectric conversion element which converts an optical signal power into by receiving optical signals at the PD surface thereof and thus is considered to be an electric current source on an electric circuit. For this reason, for using a voltage signal according to a quantity of received light, it is necessary to use a current-voltage conversion circuit. The simplest example of the current-voltage conversion circuit is parallel connection of a PD and a resistor, which can obtain a voltage signal according to a quantity of received light.